Field of the Invention
The invention concerns a method for slice-selective detection and correction of incorrect magnetic resonance (MR) image data in slice multiplexing measurement sequences, and a magnetic resonance system designed to implement such a method. In particular, the present invention concerns the individual detection and correction in individual slices given otherwise parallel imaging.
Description of the Prior Art
Magnetic resonance tomography is an imaging technique that is used for examination and diagnosis in many fields of medicine. The physical effect of nuclear magnetic resonance forms the basis. To acquire MR signals, a static basic magnetic field is generated in the examination region at which the nuclear spins (the magnetic moments) of the atoms in the examination subject align. The nuclear spins can be deflected or excited out of the aligned position (i.e. the idle state) or a different state by radiating radio-frequency pulses. During the relaxation back into the idle state, a decay signal is emitted that can be inductively detected by one or more reception coils.
The phase evolution of the spin system of a slice is described by the coherence curve (progression). If the spins of a spin system of a specific slice all have an identical phase position, this is described by a disappearing dephasing of the coherence curve. A signal can be detected since no destructive interference exists between the signals of various spins of different phase.
By applying a slice selection gradient upon radiation of the radio-frequency pulses, nuclear spins are excited only in a slice of the examination subject in which the resonance condition due to the local magnetic field strength is satisfied. An additional spatial coding can take place by the application of at least one phase coding gradient, and a frequency coding gradient can be activated during the readout. It is thereby possible to acquire MR exposures of multiple slices of an examined person. By means of suitable presentation methods it is possible to provide a 3-dimensional (3D) image of a specific region of the examined person for diagnosis.
In the clinical environment there is always a quest for faster MR acquisitions, in particular 3D acquisitions. MR measurement sequences to generate MR exposures can be optimized in this regard. In particular, MR sequences in which images are acquired simultaneously from multiple slices within the scope of multiple acquisition sequences (i.e. sequences as slice multiplexing measurement sequences) lend themselves to this. In general, slice multiplexing measurement sequences can be characterized by a transverse component of the magnetization of at least two slices being specifically used simultaneously for the imaging process, at least during a portion of the measurement. The use of magnetization for the imaging process can mean the simultaneous excitation or deflection of the magnetization, the simultaneous dephasing and rephasing (by gradient fields, for example) or also the simultaneous readout of the magnetization. In contrast to this, in the established multislice imaging, the signal of at least two slices is acquired in alternation, i.e. completely independently of one another and with a correspondingly longer measurement time (what are known as “interleaved” measurement sequences). This is essentially intended to produce a mere relaxation of the magnetization of one slice during the excitation of an additional slice, which does not directly contribute to the imaging process.
Various slice multiplexing measurement sequences are known. For example, given simultaneous excitation of the magnetization and/or detection of an MR signal the addressing of the various slices can take place via a phase coding (what is known as “Hadamard” coding; see in this regard S. P. Souza et al. in J. Comput. Assist Tomogr. 12 (1988) 1026) or a frequency coding (what is known as broadband data acquisition; see in this regard E. L. Wu et al. in Proc. Intl. Soc. Mag. Reson. Med. 17 (2009) 2678).
Furthermore, there are MR measurement sequences that use multiple radio-frequency coils to differentiate various slices. With knowledge of the spatial acquisition characteristic of the different radio-frequency coils, the simultaneously acquired data can be separated by means of suitable computing operations. Such methods are known under the names SENSE, GRAPPA or SMASH, for example, as described in D. J. Larkman et al. in J. Mag. Res. Imag. 13 (2001) 313.
An additional measurement sequence is based on the short temporal separation of the signal excitation steps and the signal detection steps. However, the gradient pulses are suitably switched so that at the same time the coherence curve of the transversal magnetization or the dephasing of the spin systems of different slices can be varied (simultaneous echo refocusing, as described in D. A. Feinberg et al. in Magn. Reson. Med. 48 (2002) 1.)). Images of both slices can be generated as is customary from the MR image data, which are acquired with hardly any temporal separation.
In the acquisition of MR image data, systematic and statistical errors in the acquisition process can generate artifacts in the MR image data. Incorrect MR image data normally cannot be used for medical diagnostics. Therefore there are numerous correction methods for the reduction of image artifacts that can be applied to single slice imaging or the established multislice imaging, in which the signals of multiple slices are acquired completely independently of one another.
For example, such correction methods concern the correction of phase errors caused by accompanying Maxwell fields. Ideal magnetic field gradients are physically unrealizable. A deviation of the spatial dependency of the magnetic field gradients relative to the linear case follows from Maxwell's equations. A correction of this Maxwell field due to phase errors is possible on the basis of calculated correction parameters. In particular, it is desirable to already apply the phase corrections slice-selectively during the acquisition process. An artifact formation due to incorrect MR data can then already be avoided during the acquisition process.
Furthermore, it is possible to correct segment-dependent phase errors. Due to statistical measurement errors, individual segments of the MR image data can exhibit phase errors. In particular, such segment-dependent phase errors are slice-specific. For example, in the literature the image artifacts produced by segment-dependent phase errors are known by the term “Nyquist ghosts”. Such phase errors or image artifacts can be computationally compensated via the measurement of reference phases. However, it is must be ensured that the detected reference data are slice-specific.
The possibilities of correction or detection of incorrect MR image data are very limited in slice multiplexing measurement sequences. In particular, due to the high degree of parallelism of the acquisition sequences of different slices there is hardly any possibility to individually affect individual slices or acquire individual slice-specific data. Correction methods that are only implemented in image space (i.e. already take place after the measurement and after separation of the slice image previously acquired in parallel) do not have the same efficiency as corrections that are applied during the measurement process itself or during the measurement sequence.
From DE 10 2009 020 661 A1 a method is known that achieves an optimally slice-specific optimization of MR measurement sequences or, respectively, a correction of incorrect MR data by means of a suitable establishment of the active volume as a set union of the volumes of all simultaneously acquired slices. However, given increasing spatial separation of the measured slice a correction implemented in such a manner suffers from a degradation since the approximation of an individual active volume no longer applies given an increasing spatial separation.